The present invention relates to a method of welding to fabricate double-wall structures composed of inner and outer wall and reinforcing ribs provided between the outer and inner walls. The invention also relates to a double-wall structure produced by the method. Double-wall structures that can be produced by the method include ship, airplane and car bodies, as well as bridge structures, various containers for use in industrial plants, and vacuum vessels.
The present invention relates particularly to a method of welding a vacuum vessel composed of a double-wall structure reinforced with ribs between the outer and inner walls. The invention also relates to a vacuum vessel produced by this welding method. The invention further relates to a method of welding a double-walled vacuum vessel for a nuclear fusion device which consists of an inner wall to be exposed to a plasma and an outer wall that surrounds it, with the walls being reinforced with ribs and welded to each other. The invention also relates to a vacuum vessel for a nuclear fusion device that is produced by this welding method.
An electron beam welding method is conventionally applied to fabricate double-wall structures such as ship, airplane and car bodies, as well as bridge structures and vacuum vesslels by welding the inner and outer walls together with reinforcing ribs.
In the class of nuclear fusion devices called "tokamak", a plasma created within a vacuum vessel is heated and sufficiently maintained with a strong external magnetic field to initiate a fusion reaction. The plasma-confining vacuum vessel is required to be capable of sustaining a high vacuum and function as a primary barrier against radiations. It is also required to withstand the large electromagnetic force produced by plasma disruption. In addition, in order to secure plasma control characteristics and reduce the loss of magnetic flux that occurs when a plasma is brought to energy break-even conditions, the electrical resistance of the vacuum vessel as measured in the toroidal direction of the torus must be held higher than a certain value. These requirements can be met by a vacuum vessel of double-wall structure that consists of an inner wall to be exposed to a plasma and an outer wall that surrounds it and in which the inner and outer walls are joined with ribs to segment the vessel wall in compartments. If the reinforcing ribs are arranged in a poloidal direction, only a small effect is caused on the electrical resistance of the vacuum vessel in the toroidal direction.
An example of this type of vacuum vessel for use in fusion devices is described in Japanese Patent Public Disclosure No. 731919/1990. The double-wall structure adopted by such vacuum vessels has the additional advantage of securing mechanical strength and rigidity. On the other hand, extensive use of welding is required to fabricate this double-wall structure. In addition, the welding of the outer wall to ribs, which is performed after inserting a shield between adjacent ribs, can only be effected from outside. Conventionally, the outer wall is externally joined to ribs by plug welding which is illustrated in FIG. 13. However, it is difficult to secure the desired weld strength by this method. If slot welding is substituted with a view to increasing the throughput, the distortion in welding is also increased and, what is more, the man-hour required is exorbitant.